Edge and latitude developer

ABSTRACT

A DEVELOPER FOR DEVELOPING ELECTROSTATIC IMAGES ON SURFACES. THE DEVELOPER CONTAINS AT LEAST TWO TYPES OF PARTICLES, ONE OF WHICH IS AN INSULATOR AND IS PREFERABLY OF FINER SIZE AND THE OTHER OF WHICH IS A MORE CONDUCTIVE PARTICULATE DEVELOPER AND PREFERABLY OF COARSER SIZE WITH POSSIBLY A FURTHER PARTICULATE MATERIAL WHICH HAS ONE END INSULATING AND THE OTHER RELATIVELY MORE CONDUCTIVE OR POLAR WHEREBY THE MORE CONDUCTIVE PARTICLES TEND TO DEVELOP THE EDGES OF THE IMAGE AND THE INSULATING PARTICLES THE INTERMEDIATE PORTIONS BETWEEN THE EDGES BUT WHEREIN THE INSULATOR PARTICLES ENABLE THE RELATIVELY MORE CONDUCTIVE PARTICLES TO BE RETAINED ON A SURFACE BY A LIMITING CHARGE TRANSFER OF THE RELATIVELY MORE CONDUCTIVE PARTICLES TO THE SURFACE CONTAINING THE THE ELECTROSTATIC IMAGE, AND THE THIRD TYPE OF PARTICLE CAN ROTATE AS A STIRRER.

Ct. 16, 19Y3 L K A METCALFE ET AL 3,766,972

LDGH AND LA'IL'IUDE DEVELOPER Original Filed Dec. 20, 1969 U nitcdStates Patent U.S. Cl. 252-621 6 Claims ABSTRACT OF THE DISCLOSURE Adeveloper for developing electrostatic images on surfaces. The developercontains at least two types of particles, one of which is an insulatorand is preferably of finer size and the other of which is a moreconductive particulate developer and preferably of coarser size withpossibly a further particulate material which has one end insulating andthe other relatively more conductive or polar whereby the moreconductive particles tend to develop the edges of the image and theinsulating particles the intermediate portions between the edges butwherein the insulator particles enable the relatively more conductiveparticles to be retained on a surface by a limiting charge transfer ofthe relatively more conductive particles to the surface containing theelectrostatic image, and the third type of particle can rotate as astirrer.

CROSS-RELATED APPLICATION This application is a continuation ofcopending application Ser. No. 889,138 filed Dec. 30, 1969, and nowabandoned.

SUMMARY OF THE INVENTION This invention relates to electrostaticprinting and in particular to the development of electrostatic images bythe deposition of particles from gases or liquid vehicles. It is nowwell known to develop electrostatic images by subjecting them to a bodyof gas or liquid containing suspended particles and to cause theparticles to be attracted to the surface of the receiving sheet in imageform.

Electrostatic images produced by X-ray or gamma ray means requirecontinuous tone development while at the same time they require highcontrast and sharp delineation of line images. These requirements areconflicting and require special development.

One object of the present invention therefore is to produce a developerwhich operates so as to give high contrast images where there are sharpchanges of X-ray intensity such as occur for a sudden change ofthickness, and also simultaneously give a very low contrast for slowchanges of X-ray intensity. These features enable details to be shownwith a high contrast over a wider range of thicknesses in the objectbeing examined than can be done with normal radiography.

Normal electrostatic images however require a more uniform developmentand edge effects require to be avoided where for instance a uniformvoltage requires to be developed with a uniform deposition.

A further object of the invention therefore is to allow control ofdevelopment to allow this uniform development to take place if this isrequired.

Another object of the present invention is to prevent deposition ofdeveloper in background non-image areas.

These objects are achieved by incorporating in the developer at leasttwo main groups of developer materials, the first of which are relativeconductors, or semicon- 3,766,072 Patented Oct. 16, 1973 ductors foredge development, the other being insulating particles which willmigrate in the electric field and be attracted to areas to be retainedat such area.

Thus chain forming particles may be used which comprise pigment-resinaggregates made by precipitating resins or the like in solvents of lowsolvent power for these resins and comprising particles of a range ofparticle sizes. The aggregate size is controlled by the addition of adispersant or solnbilizing substance which is more soluble than thefirst resin in said solvent of low solvent power and is designed toincrease or reduce the size of the aggregates of the first resin becauseof its solubility for the first resin and its miscibility with thesolvent or because of its wettability for the particles of the firstresin.

BRIEF DESCRIPTION OF THE DRAWING For better understanding of theinvention reference should be made to the accompanying drawings inwhich:

FIG. 1 illustrates a series of tests made with both insulating andconductive materials and compound insulatorconductor materials whensuspended in an insulating fluid and subjected to a unidirectionalfield,

FIG. 2 shows diagrammatically how a sheet of photoconductive materialbearing a latent electrostatic image can be developed to give forinstance an edge effect, and

FIG. 3 shows a particle of developer showing how it can be coated tocontrol the required action. The particle could be conductive and thecoating insulating, or vice versa.

DETAILED DESCRIPTION Referring first to observations we have made of thebehavior of pieces of insulating material, relatively conductivematerial, and compound pieces, suspended in insulating liquids throughwhich an electric field is applied as indicated in FIG. 1.

In the example tested, an insulating liquid 1 was prepared whichcomprised a mixture of Freon 113 and Isopar H. In this liquid a piece ofinsulating material 2 was suspended between a pair of electrodes 3 and 4so that it floated in the liquid. An electric field in the range 1000volts per inch to 10,000 volts per inch was applied and it was notedthat the insulating piece migrated to one electrode and remained thereas shown in A of FIG. 1.

In the second test we used a conductive piece 6 or a semiconductor pieceand also we used insulating material coated over its whole surface witha layer of conductive material such as graphite, or wetted with arelatively conductive material such as water or glycerol triacetate.Such a conductive piece will migrate to the electrode nearest to it atthe time of application of the field and then dissipate its charge andthen acquire the same polarity of charge as the electrode and berepelled by this electrode and will oscillate between the electrodes.This is shown in B of FIG. 1.

In a third situation as shown in C of FIG. 1, the insulating piece 7 iscoated with conductive material on one side only and it is found thatthe piece will migrate to the nearest electrode with the conductive ofhigh dielectric constant side turning to a foremost position. Contact ofthe conductive end with the electrode results in repulsion of the piecewhich then migrates to the op posite electrode, again turning over, andthe process is repeated.

It is important to note at this stage that the period of oscillation inthe case of pieces B and C of FIG. 1 can be varied by either insulatingthe particle or the electrode, the thickness of the insulationcontrolling the time it takes to reverse the charge on the insulator.Thus a thin insulator increases the speed of reversal of direction ofmigration of the particle and thus where normally a conductive piecewould oscillate, it is found that by inserting a thin insulating filmbetween it and the electrode which it is approaching, results in delayin sharing of charge with the electrode and consequently, delay inrepulsion. A similar insulating piece coated with a layer ofsemiconductive material migrates backwards and forwards in a similar wayto the conductive piece but is characterized in that it pauses inoscillation after arrival at each electrode as if an insulating barrierexisted on the electrode. Thus it follows, and this is an importanteffect of this invention, that while insulator particles can be readilybuilt up on a latent image, conductive particles can first be moved downand then repulsed, so that the degree of insulation or conductivity nowallows a selection of the type of effect desired. By mixing theparticles agitation effects on the insulators can also be achieved.

Thus insulator particles can be more readily retained on a chargedsurface and will continue to build up according to charge level.Conductive particles will however migrate onto and then off the surfacebut the rate can be controlled by an insulating film.

Such an insulator film could be built up on the image by fine insulatorparticles in the developer to then allow large conductive orsemiconductive particles to be retained due to these particles thenbeing prevented from rapidly transferring charges they have acquired.

Conductive particles are more strongly attracted to a field ofsutficient intensity but have a quicker charge transfer and leave thearea quicker if the field strength is increased.

Non-conductive particles move slower in the field but when they deposit,provided the field is strong enough, they are held in place.

It must be appreciated that conductive particles when deposited on asurface allow lateral spread of the field, and thus by short developmentthe condition of FIG. 2, namely edge development is achieved.

If development time is extended the conductive particles held in theweaker field areas extend the field and this field straightening efiectthen allows relatively uniform development to occur.

if ions or migrating particles of substantially different size andpolarizability are injected into the system, for example by addingiodine, metal naphthenates or other chain forming compounds whichconstrict the field, there is a signicant change in the type andvelocity of migration of the particles or pieces, conductive pieces tendto remain stationary, insulating pieces tend to migrate to one electrodeand stay there, pieces with regularly arranged surface spots ofconductive coating also tend to remain stationary whereas pieces withconductive coatings predominantly on one end rotate in a stationaryposition. This is shown in D in FIG. 1.

In either system particularly where the pieces or particles have a highdielectric constant a situation readily arises in which fine articleswill coat on to coarse particles even should the basic material be ofthe same composition. Mechanical agitation also can effect depositionand coating. Thus it is seen that the migration of pieces depends ontheir composition, conductivity, dielectric constant, anddistribution ofsurface properties as well as the magnitude and direction of theelectric field.

All of the effects observed in these experiments show that there isincreased activity in the cell as the electric field is increased. Theseexperiments demonstrate that particle behavior as induced by highelectric field intensity is not predictable by known particle transportmechanisms such as an electrophoresis or dielectro phoresis.

We have applied these phenomena to enable deposition to be made inaddition to the conventional technique which provides continuous tonedevelopment. Thus the conductive particle which is shown to oscillatebetween the conductive electrodes and also to adhere to an in- 4sulated'electrode is used in the developer to provide edge accentuationof the developed image.

The third experiment in which an insulating piece is used having acoating of a conductive material at one end is analogous to anaggregated resin which has been initially dissolved in a polar orrelatively conductive solvent. In the case of development ofradiographic images the abrupt changes in section of an object beingradiographed produces an electrostatic image characterized by a highpotential difference across the section and consequently a localizedhigh field intensity.

From these observations we have learned that in order to carry out thepresent invention it is necessary to produce aggregates of particleswhich will be attracted initially only to a relatively strong electricfield and therefore tend to exaggerate edges of images when these imagesare developed face up, but in addition the developers must containparticles of sufiiciently small size, or particles of non-polar type,which will develop weak image fields.

In one form of this invention the aggregate is obtained by precipitatinga resin from solution in a good solvent by putting this solution into apoor solvent. For example if the resin is dissolved in an aromatichydrocarbon, precipitation is obtained by injecting the solution into analiphatic hydrocarbon containing various percentages of aromatichydrocarbon for example from zero to 50% and it is found that theaggregate size increases with lower aromatic content until totalprecipitation occurs, depending on the type of resin. If it is desiredto produce a final developer in a hydrocarbon solvent containing zeroaromatics, the solubility of the solvent for the resin and hence theaggregate size may be refined by means of other resins and solubilizingagents which have an aflinity for both the final resin and also thesolvent.

Pigmentation or coloration of the aggregates is achieved byincorporating a pigment or dye in the original resin solution prior toprecipitation.

The fine material necessary for development of small charges or fields,is obtained either by parting of particles from aggregates or clustersor by separate additions to the concentrated developer of fine particlesize material.

The following examples will serve to illustrate the invention:

EXAMPLE 1 A developer concentrate is prepared by first dissolving arelatively soluble resin such as Solprene 1205 in an aromatichydrocarbon solvent such as Esso and also a relatively insoluble resinsuch as V.T.A.C. (vinyl toluene acrylic copolymer) in a similar solventand milling these together with a colorant such as Microlith Black CTand a small proportion of Microlith Blue. The proportions of theSolprene 1205 in relation to the V.T.A.C. may for example be 3 parts ofSolprene to 2 parts of V.T.A.C. by weight, and the proportions ofpigment to those of the total resin may for example be 1 part of pigmentto 1 part of resin by weight. A typical formulation is as follows:

Grams Solprene 1205 resin (styrene-butadiene resin of Phillips Corp.,USA.) Pliolite (V.T.A.C. resin, Goodyear Corp., USA.) 60 Esso 100solvent 200 Microlith Black pigment (C.I.B.A.) Microlith Blue pigment(C.I.B.A.) 25

The developer concentrate made in this way is then dispersed in analiphatic hydrocarbon solvent such as Isopar G with vigorous stirring toform aggregates of particles in the proportions of 1 gram in 100milliliters. It will be clear that as the amount of concentrate isincreased in initial proportion to the carrier liquid that changes willoccur in aggregate size because of increasing solvent power of the totalliquid. The preferred range is 0.5 to 2 grams per 100 milliliters.

In the above example it should be noted that the styrene butadiene resinis a solvent for the V.T.A.C. resin and these-co-deposit in the Essosolvent as a fine developer of insulating particles.

The Microlith Black pigment has relatively conductive particles and theMicrolith Blue somewhat poorer conductive particles.

The insulating resin particles do not Wet the conductive particles butco-exist in the insulating carrier liquid so that the conditionsillustrated in MG. 2 are present and short term development of a latentimage tends to accentuate edgeeifects as the larger conductive particlestend to build up quicker in the stronger edge fields but as developmentcontinues the conductive particles deposited on the insulator particlesat the originally weaker field give lateral spread to develop more onthe basis of the voltage gradient shown. In FIG. 2 a vessel 8 has in itthe insulating carrier liquid 9 and a photoconductive sheet is shown onwhich is a latent image indicated by the voltage gradient line 11. Thefieldgradient is shown by the dotted line 12.

The particle of FIG. 3 is designated 13, and has on it a film 14.

EXAMPLE 2 A typical formulation is as follows:

. Grams Solprene 1205 resin l6 Mineral turpentine solvent 40 Added to:

Elvacite 2010 resin 4 (The Solprene resin is the solvent for theElvacite resin.)

T richlorethylene 4 And then added to:

Microlith Black CT (C.I.B.A.) 17

Microlith Green (C.I.B.A.) 3

Both of these are relatively conductive but are lightly coated and thusare partly insulated.

EXAMPLE 3 A developer concentrate is prepared as in Example 1 but thepigment is replaced by Coates Hydrocarbon dispersible flake black. Thispigment is more conductive than the pigments of Example 1.

EXAMPLE 4 A developer concentrate is prepared as in Example 2 but theElvacite (methyl methacrylate) resin is replaced by a Melamine resin.This is even more insoluble than the Elvacite resin and thus results incoarser insulating particles.

. EXAMPLE 5 A developer concentrate is prepared as in Example 1 but theSolprene 1205 resin is replaced'by Pliolite SSD (Goodyear Corp.,U.S.A.). This again results in grain coarseningbecause it is a poorersolvent for the other resin.

EXAMPLE 6 i A developer concentrate suitable for use in Isopar G or Ecarrier liquidsis prepared from the following materials:

Grams Brillfast Black pigment 24 Solprene 1205 resin 18 Ethyl hydroxyethyl cellulose resin 6 Solvesso solvent 48 These materials are blendedin a bar mill at 40 psi. The Brillfast pigment is more conductive thanthe coated black pigment.

EXAMPLE 7 To Example 1 is added:

.005 by weight of solids of zirconium octoate or .001% of cobalt octoateor .001% of iodine, or

.001% of lead naphthenate to provide rotating particles.

IDENTIFICATION OF TRADEMARKS AND TRADE NAMES Solprene 1205 is a blockcopolymer of butadiene and styrene in the ratio of 75/25 manufactured bya solution of polymerization process by Phillips Corp., U.S.A., A.S.T.M.No. 1205, characterized in that the majority of the styrene moleculesare added as polystyrene at the end of a long chain of butadiene units.

Pliolite VT Resin is a styrene/butadiene type copolymer rubber made bythe Goodyear Corp., U.S.A. and prepared by the G.R.S. method in whichthe butadiene polymerizes in the main by a 1,4-addition. Pliolite VT isa vinyl toluene/butadiene random copolymer rubber, soluble in mineralspirits.

Pliolite SSD is a styrene/butadiene copolymer, KB value 60, manufacturedby Goodyear Corp., U.S.A.

Pliolite V.T.A.C. is a vinyl toluene/acrylate copolymer, KB value 36.

Esso 100 Solvent is a hydrocarbon solvent supplied by Esso ChemicalsAustralia Limited, having an aromatic content of 98%, flash point of 108F., and distillation range 159182 C.

Microlith Pigments comprise a pigment and a resinous carrier. MicrolithBlack pigment contains pure neutral carbon black together with a toluenesoluble carrier resin such as Stabilite Ester 10 of the Hercules PowderC0., U.S.A.

Microlith Blue 4GT comprises a stable phthalocyanine blue pigment with agreenish cast together with Stabilite Ester 10 resin.

Microlith Green GT comprises a medium shade of phthalocyanine greentogether with Stabilite Ester l0 resin, the microlith pigments aremanufactured by Ciba C0., Switzerland.

Color Index of the pigments:

Microlith Blue-Color Index No. 74160 Microlith Green-Color Index No.72455 Elvacite Resin referred to in Examples 2 and 4 is an acrylic resinmanufactured by Du Pont, Delaware, U.S.A.

Coates hydrocarbon dispersible flake black comprises pure carbon blacktogether with ethyl hydroxy cellulose resin.

What is claimed is:

1. A developer for developing electrostatic images on a surfacecomprising a carrier liquid medium having an electrical resistivitysutficiently high to prevent destruction of a latent electrostatic imageduring development, developer particles suspended in said carrier liquidmedium and including at least two types of particles, the first of saidtwo types being insulator particles and having an inherent charge butbeing electrical insulators unable to exchange charges with the surfaceduring development and the second of said type types being relativelymore conductive polar particles whereby charges can be exchanged with alatent image bearing surface, the insulator particles adhering to asurface having an appropriate image field polarity without repulsionbecause of the inability to transfer charges, the relatively moreconductive polar particles acquiring the image field polarity andexchanging charges with the surface to accept the surface image chargedto be repulsed thereby by acquired similar charge characteristics unlessinsulated from the surface by attracted insulator particles, saidinsulator particles being smaller than the more conductive polarparticles whereby the insulator particles tend to deposit on lower imagecharge areas, the more conductive polar particles tending to depositwith some of said insulator particles on higher image charge areas, themore conductive polar particles being chain forming and comprisingpigment-resin aggregates which are precipitation products of resins incombination with pigments in solvents of low solvent power for theresins.

2. A developer according to claim 1 including ions or migratingparticles of substantially different size and polarizability selectedfrom the group consisting of iodine, metal naphthanates and metaloctoates in the developer to reduce the velocity of migration of thedeveloper particles.

3. A developer according to claim 1 including added insulatingparticles, having one side conductive comprising aggregated resin whichhas been initially dissolved in a polar or relatively conductivesolvent, and including ions or migrating particles of substantiallydilferent size and polarizability selected from the group consisting ofiodine, metal naphthanates and metal octoates in the de- 8 veloper tocause the added insulating particles to rotate within the developercarrier fluid.

4. A developer according to claim 1 wherein the insulator particlescomprise pigment particles coated with a block copolymer of butadieneand styrene in the ratio of about percent butadiene and 25 percentstyrene, the styrene molecules being in the form of polystyrene at theend of long chain butadiene units.

5. A developer according to claim 1 wherein the insulator particlescomprise pigment particles coated with a styrene/butadiene copolymer.

6. A developer according to claim 1 wherein the insulator particlescomprise pigment particles coated with a vinyl toluene/acrylatecopolymer.

References Cited v UNITED STATES PATENTS Ricker 252-62.1X

ROLAND E. MARTIN, Primary Examiner

